A standard bumper assembly mounted on the front or rear of a motor vehicle comprises a relatively rigid and quite strong rail extending horizontally and transversely, that is cross-wise to the normal motor-vehicle travel direction and normally carrying the plastic or sheet-metal elements forming the visible parts of the bumper system. This rail is secured to the motor vehicle frame by at least two mounts that themselves therefore extend longitudinally, that is parallel to the normal motor-vehicle travel direction, and that have front ends fixed to the rail and rear ends fixed to the motor-vehicle frame. It is known to provide towing points, typically sockets to which a tow line can be attached, on the mounts. These mounts are sacrificial and thus are constituted as so-called crush cans so that, in the event of a low-speed collision, they deform longitudinally, confining the damage to these elements. Thus JP 02175452 describes how they can be formed with cross-wise grooves so that, when crushed longitudinally, they deform uniformly and absorb considerable energy.
In U.S. Pat. No. 6,439,650 of Artner the crush cans each comprise two vertical side walls, a horizontal top wall, and a horizontal bottom wall, with the top and bottom walls horizontally bridging upper and lower edges of the side walls. The top wall is generally level with and extends rearward or forward, depending on whether this is the front or rear bumper, from an upper surface of the rail, and the bottom wall is similarly generally level with and extends rearward or forward from a lower surface of the rail. Thus the crush cans are in effect wholly tucked in behind the rail.
The problem with this system is that the space between the stiff rail and the vehicle frame is fairly limited. In addition it is frequently uses as a flow path for cooling air into or out of the vehicle's engine. Thus the crush cans cannot be too big, as on the one hand there is insufficient room to accommodate something big, and on the other hand the space cannot be filled up and blocked. Attempts to reduce the size of the crush cans leads, however, to reductions in the strength of the cans, so that they cannot meet, for instance, the European low-speed crash requirements which are at 16 km/h.